[TCP] Vegas: timestamp before clone

We have to store the congestion control timestamp on the SKB before we
clone it, not after.  Else we get no timestamping information at all.

tcp_transmit_skb() has been reworked so that we can do the timestamp
still in one spot, instead of at all the call sites.

Problem discovered, and initial fix, from Tom Young
<tyo@ee.unimelb.edu.au>.
Signed-off-by: David S. Miller <davem@davemloft.net>